As this type of a conventional steering lock device, one that is disclosed in Patent Literature 1 (Japanese Patent Application Laid-Open Publication No. 2004-231122) is known. FIG. 1 shows a structure of a steering lock device described in Patent Literature 1. This steering lock device 101 is provided with a lock member 150 for prohibiting rotation of a steering shaft 180. Moreover, the lock member 150 is movably arranged between a steering lock position at which the tip of the lock member 150 protrudes from a guide part 112 of a frame 110 and a steering lock release position at which the tip of the lock member 150 is accommodated within the guide part 112. Further, this steering lock device 101 is provided with a breaking structure 170 as a protection mechanism for a driver in a collision accident or the like. The breaking structure 170 is composed of a groove-shaped fragile part 171 formed on a hanger 152 that constitutes the lock member 150 and a groove-shaped coupling part 172 formed on the frame 110. The fragile part 171 is formed at a position of the hanger 152 that overlaps with the coupling part 172 in a state that the lock member 150 is positioned at a steering lock release position. The breaking structure 170 restrains entry of reaction force to an on-vehicle load by breaking when excessive external force is applied such as by the on-vehicle load striking against the steering lock device 101 in a collision accident or the like. Furthermore, this steering lock device 101 is provided with an auxiliary lock mechanism 160. When the breaking structure 170 breaks, an auxiliary lock member 161 engages with a lock body 151 that constitutes the lock member 150 to retain the lock member 150 in the steering lock release position.